1. Field of the invention
This invention relates to a method and apparatus for correcting a widthwise bend in an end portion of a hot-rolled elongate sheet-shaped metal product, the correction being performed while the product is still hot from the hot-rolling or after reheating of the product. The invention further relates to a hot strip mill including straightening apparatus for correcting such a bend. The invention is particularly applicable to hot-rolled products of steel and aluminium. The products may be intermediate products or final products.
2. Description of the Prior Art
In the hot-rolling of steel or aluminium products, typically a hot ingot is first passed several times through a roughing mill which forms it into a so-called bar, which is an elongate sheet-shaped product. Typically this bar has a width of 900 to 1500 mm, a thickness of 20 to 80 mm and a length of for example, 50 m. This bar, when intended for further rolling into a strip, is fed into a finishing mill comprising a number of roll sets and is coiled as a strip having a thickness of, for example, 2 to 6 mm, while still hot. Alternatively, the product from the roughing mill may take the finished form of a so-called sheet or plate.
All of these products from the roughing mill may suffer from the defect of a lateral bend in their head end portion or their tail end portion. The bend in the head end portion is usually larger, and causes greater difficulties, particularly if the product is a bar which is subjected to finishing rolling as described above. The bent head end causes problems during threading into the finishing train, which can lead to non-uniform thickness of the rolling in the finishing train and also a problem of folding of the leading end, as well as possible difficulties in coiling.
One solution to the problem is to cut off the bent portion, but since this may be 4 m long, it amounts to perhaps 8% of the product, which is wasteful.
One proposal in the prior art has been to correct the bend during the roughing rolling, using edger rollers which engage the edges of the bar during a rolling pass through the roughing mill. These edger rollers have vertical axes which are adjustable laterally of the bar and remain fixed during a rolling operation. A problem particularly caused here is that the edger roller presses with high force against the bar, causing thickening of the bar at that side, with consequent further problems during rolling.
An alternative possibility, not disclosed in the prior art, is to adjust the rollers of the roughing mill during a rolling pass, so that they roll the bar non-uniformly, in a manner which corrects the bend. This method however is thought to be not practical, on account of the high investment cost in designing the mill to perform this action.
In a hot-rolling mill, fixed side guides are disposed at several locations on the conveyor table for the hot-rolled bar, e.g. as illustrated in the Japanese-language Iron & Steel Handbook, page 416, for example. These side guides are fixed during rolling, but may effect some straightening of a bar having a widthwise bend simply by pressing upon the bar. However, the amount of straightening is small, because such guides are designed with a clearance between them and the edges of the bar. Typically this clearance is about 50 mm and is necessary because the edge of the strip is not straight. Accordingly, such a side guide cannot effect correction of a bend except perhaps over a very long length of the bar.